This invention relates to the field of seismic signal processing, and specifically to the area of three dimensional seismic signal processing.
In performing traditional two dimensional seismic data processing on multiple fold data, common mid-point ("CMP") gathers are made. In comparing the traces in the gather, the offset (distance between the source and receiver) of the traces varies. Further, in comparing the gather of one mid-point to the gather of another midpoint, the number of traces and the offset variation is substantially the same. Most differences occur due to the need to remove an obviously bad trace from the data set. However, in high fold data, such blanking is not appreciable.
In performing three dimensional analysis, rather than common mid-point gathers, common mid-point bins are made of the data, which include traces having a common midpoint, and various offsets from ray traces having traveled cross-line. Such bins might have consistent fold, but uniform offset distribution does not exist. For example, as seen in FIG. 1, a typical acquisition geometry for ocean-bottom prospecting is seen, in which two receiver lines RL1 and RL2 are laid out parallel to each other. Sail Lines are shot orthogonal to the receiver lines at regular intervals (SL1).
Referring now to FIG. 3, nine common-midpoint bins (BIN 1-BIN 9) from the survey geometry of FIGS. 1 and 2 are shown, in which each line within the bin represents a trace, the vertical and horizontal axes are offset. Here, it is seen that the offset distribution is not uniform. This pattern is dependent upon the acquisition geometry, and this non-uniform pattern has not been found to be avoidable. Changing the acquisition geometry to accommodate offset distribution in the common mid-point bins is not practical.
In some forms of analysis, the variation of trace attributes as a function of offset or angle of reflection is of interest (e.g., AVO, AVA, and other offset-dependent-reflectivity analysis). However, as seen in FIG. 4, where one of the offset bins of FIG. 3 is seen divided into multiple offset bins OB1-OB8, the offset is so non-uniform that offset bins OB1 includes only one trace and bin OB7 contains thirteen. When the traces within the offset bins are stacked, the large variation detrimentally affects the analysis. This occurs because the variations created in normalizing the amplitude and noise components of the data, after stacking such non-uniform fold, influences one of the very attributes to be studied--amplitude.
Accordingly, there is a need for a method of providing common-offset bins, within a common mid-point bin, which are uniform in distribution.
In conducting amplitude variation with offset analysis ("AVO") and amplitude variation with angle analysis ("AVA"), in three dimensional data sets, it is common to analyze the amplitude in a CMP only relative to offset, in a two-dimensional fashion, and assign a value or slope to the variation within that bin. No azimuth or directional information is preserved that would indicate the trend of variation within the bin. Therefore, amplitude variation trends across a 3D survey are not conducted, and there is a need for a method of conducting AVO and/or AVA analysis in which trend information within the survey bins is available.